1. Field of the Invention
This invention relates to the field of energy. More specifically, the invention comprises a modular energy harvesting system, suitable for harvesting energy from available heat sources.
2. Description of the Related Art.
Organic Rankine cycle heat engines are well suited to the recovery of energy from relatively low temperature heat sources. Such heat sources include biomass reactors, concentrated solar collectors, engine exhaust, industrial waste heat, heat extracted from mines, and geothermal sources such as deep crystalline bedrock. A particularly promising example is “enhanced” geothermal energy. This example will be used for the disclosure of the present invention, but the reader should bear in mind that the invention is equally applicable to many other heat sources.
Enhanced geothermal energy systems seek to extract energy from deep “hot dry rocks” by circulating a working fluid (typically water) through the rock. Once a suitable site is located, an injection well is drilled down into the rock strata. Such a well may be drilled as deep as 4,000 to 8,000 meters below the surface.
One or more return wells are drilled down into the same strata. Hydraulic fracturing may be used to fracture the strata so that water may more easily pass through from the injection well(s) to the return well(s). Proppant material may also be added to the strata to maintain the desired flow rates.
A working fluid is circulated through the rock strata. For deep strata this will typically be water. Pressurized water is pumped down through the injection well(s). It seeps through the strata and then returns to the surface via the return well(s). In passing through the rock strata the water absorbs considerable heat. As an example, water pumped in at an ambient temperature of 17 degrees C. may emerge at 180 degrees C. or more. Thus, the circulating water has carried some of the thermal energy found at great depths within the earth to the surface.
Geothermal heat is available over a wide temperature range. Those knowledgeable in the field of thermodynamics know that the operation of a hest engine is more dependent on the temperature spread between the boiler and the ambient conditions surrounding the condenser than it is on the absolute temperature of the heat source. Thus, in cold climates, a heat source of only 60 degrees C. is still useful because the condenser may be rejecting heat to a −10 degrees C. environment.
Of course, a 180 degree C. heat source contains more available energy. Even so, it is not hot enough to be suitable for traditional steam-based energy harvesting cycles. An organic Rankine cycle engine, however, is able to effectively utilize a heat source in this temperature range. Accordingly, organic Rankine cycle engines have been proposed for use with enhanced geothermal sources.
FIG. 1 schematically depicts such an arrangement. Organic Rankine Cycle (“ORC”) engine 10 may include multiple turbine stages, reheat, and other components known to those familiar with this technology. The version shown is the simplest embodiment. A working fluid is vaporized in boiler 20 then fed through turbine feed line 36 to turbine 22. The mechanical energy extracted by the turbine spins generator 24 (or in some instances may provide direct mechanical power). Turbine exhaust line 42 takes the turbine exhaust into condenser 26, where it is condensed back into liquid form. Feed pump 18 pulls the liquid out of the condenser through condenser suction line 38 and compresses it to the boiler operating pressure before feeding it out through boiler feed line 40. Thus, the ORC engine is a circulating loop whereby energy is harvested from an available heat source.
The heat source in this particular example is geothermal energy. The term “boiler” is used for element 20 in FIG. 1, since this is the traditional term used for a heat engine cycle. One may conventionally think of a combustion-based heat source being used in conjunction with a boiler. This is not the case for geothermal applications. The heat source for geothermal applications is hot water. Thus, boiler 20 may also be referred to as a heat exchanger in which heat is transferred from a hot, pressurized working fluid to the vaporizing refrigerant used in the ORC engine.
The hot working fluid is provided by geothermal energy. Certain geological formations provide a good source of useful heat. In this example, the heat source is a very deep and dry rock formation which has an overlying insulating layer of sedimentary rock. Injection pump 16 pressurizes the working fluid (typically water) and feeds it down injection well 28. It is injected into, a layer of hot dry rock 12 which may be as much as 4,000 to 8,000 meters below the surface.
The injected water seeps through fractured passage 14 and eventually into return well 30 which returns it to the surface. The large water passage shown does not represent the true nature of the water path. In reality, the water will flow through a complex path of small fissures. As it flows, the water is heated by the surrounding rock. By the time the water emerges from return well 30, it could be heated to over 200 degrees Centigrade. This water is then fed through boiler 20 where it transfers heat to the working fluid in the ORC engine.
A single, injection pump 16 is shown, but those skilled in the art will know that two or more pumps maybe used at various locations in the circulation loop of the enhanced geothermal well system. Likewise, it is common to use multiple injection wells and multiple return wells.
Water is currently the preferred working fluid for the geothermal circulation loop. However, other working fluids are certainly possible. Supercritical carbon dioxide is now being investigated as a possible working fluid.
The working fluid within the cyclic ORC engine itself must be selected to have appropriate phase change points in comparison to the temperature of the available heat source. The following table presents examples of ORC working fluids along with a listing of some of their physical properties:
TABLE IMol. Wt.F.P.B.P.Common NameIUPAC Name(g/mol)(° C.)(° C.)R-22chloro-difluoro methane86−135−40.8R-1141,2-dichloro-1,1,1,2-171−943.6tetrafluorethaneR-133a1-chloro-1,1,1-trifluoro118−1066.9ethaneR-134a1,1,1,2-tetrafluoro-102.03—−26.5ethaneammoniaammonia17.03−77.7−33.4toluenemethyl benzene92.1−95110butanebutane73.1−138.4−0.5Dowtherm E0-dichlorobenzene166−48Genetron 245fa1,1,1,3,3-pentafluoro-13415.3propane
The more toxic candidates—such as toluene—are not likely to be used in a large application where an accidental leak is always a concern. Thus, relatively benign compounds such as R-134a and Genetron 245fa are more likely.
The, use of hydraulic fracturing to improve the performance of enhanced geothermal sites can produce unwanted seismic activity. The injection of proppants may in some instances be undesirable near populated areas. Thus, enhanced geothermal energy harvesting plants are often located in remote areas.
It is also difficult to predict the performance of the injection and return wells due to the difficulty in determining the porosity of rock strata thousands of meters below the surface. Thus, it is advisable to test the performance of one or two small wells before committing to establishing a large-scale facility. From these considerations the reader will understand that an ORC energy harvesting device suitable for use in the enhanced geothermal energy field is preferably scalable. The present invention provides just such a device.